Sex Differences in 2-DOF Human Ankle Stiffness in Relaxed and Contracted Muscles

Abstract

Ankle stiffness has been known as one of the most important components contributing to the maintenance of lower body stability during postural balance and locomotion. It has been repeatedly shown that women have lower stability and increased risk of injury when compared to men participating in similar sports activities, yet sex differences in neuromuscular control of the ankle, including the modulation of ankle stiffness, and their contribution to stability remain unknown. To identify sex differences in human ankle stiffness, this study quantified multi-dimensional ankle stiffness in 20 young, healthy men and 20 young, healthy women over a range of ankle muscle contractions, from relaxed to 20% of maximum voluntary co-contraction of ankle muscles. A wearable ankle robot and a system identification method were used to reliably quantify ankle stiffness in a 2-dimensional space spanning the sagittal plane and the frontal plane. In all muscle activation levels, significant sex differences in ankle stiffness were identified in both the sagittal and frontal planes. In the given experimental conditions, ankle stiffness in males was higher than females up to 15.1 and 8.3 Nm/rad in the sagittal plane and the frontal plane, respectively. In addition, sex differences in the spatial structure of ankle stiffness were investigated by quantifying three parameters defining the stiffness ellipse of the ankle: area, aspect ratio, and orientation. In all muscle activation levels, a significant sex difference was identified in the area of stiffness ellipse as expected from the sex difference in the sagittal and frontal planes. However, no statistical sex difference was observed in the aspect ratio and orientation, which would be due to little differences in major anatomical configurations of the ankle joint between sexes. This study, in combination with future studies investigating sex differences during dynamic tasks (e.g. postural balance and locomotion) would serve as a basis to develop a risk assessment tool and sex-specific training programs for efficient ankle injury prevention or rehabilitation.

Sex Differences in Human Ankle Stiffness During Standing Balance

“…Previous studies have demonstrated that the incidence of musculoskeletal injuries at the ankle joint in females is significantly higher than in males participating in similar sports activities (Elias, 2001;Ristolainen et al, 2009). Among the risk factors contributing to this higher incidence of musculoskeletal injuries in females, the neuromuscular control of stability has been identified as one of the potential factors contributing to the sex difference in risk of injury (Granata et al, 2002a,b;Trevino and Lee, 2018). With ankle stiffness recognized as a major contributor of neuromuscular control of stability (Loram and Lakie, 2002;Lee and Hogan, 2015), one recent study investigated sex differences in 2D ankle stiffness in a seated position (Trevino and Lee, 2018).…”

“…Among the risk factors contributing to this higher incidence of musculoskeletal injuries in females, the neuromuscular control of stability has been identified as one of the potential factors contributing to the sex difference in risk of injury (Granata et al, 2002a,b;Trevino and Lee, 2018). With ankle stiffness recognized as a major contributor of neuromuscular control of stability (Loram and Lakie, 2002;Lee and Hogan, 2015), one recent study investigated sex differences in 2D ankle stiffness in a seated position (Trevino and Lee, 2018). Extending this previous study (Trevino and Lee, 2018), we identified sex differences in 2D ankle stiffness in the sagittal and frontal planes during upright standing balance under various task conditions: varying ankle muscle co-contraction, weight-bearing, and ankle torque generation tasks.…”

“…However, there is still little information regarding the sex differences in ankle stiffness. Only one study investigated sex differences in 2D ankle stiffness in the IE-DP space, but it was limited to a static seated position (Trevino and Lee, 2018). While this study has provided an important baseline to understand sex differences in ankle stiffness, it is unknown if results obtained in the non-functional seated tasks would be applicable to functional tasks, such as standing balance and walking.…”

Quantifying Age-Related Differences of Ankle Mechanical Properties Using a Robotic Device

“…Robotic devices permit intensive, controlled, and tailored rehabilitation, as well as reducing the therapist's burden [3]. Since the ability of locomotion is fundamental to avoid the worsening of the quality of life [4], one of the main challenges in the robotic field is the design and development of robots for ankle rehabilitation [5]; in fact it is well-known that the ankle joint plays essential roles during walking, such as shock absorption, propulsion, lower limb coordination, adaptation to different environments, and maintenance of stability [6]. From this perspective, an appropriate design of robotic devices for the ankle joint is required for: (i) rehabilitating people affected by neuromuscular diseases [7,8]; (ii) restoring athletes after injuries [9]; and, (iii) augmenting human strength and endurance in industrial and military applications [10,11].…”

How Compliance of Surfaces Affects Ankle Moment and Stiffness Regulation During Walking

“…Previous studies have shown that alterations in ankle muscle excitations can change ankle stiffness ( Trevino and Lee, 2018 ; Whitmore et al, 2019 ; Wind and Rouse, 2020 ). Our results showed that excitations of SOL and GAL on the surface with the lowest level of compliance were significantly larger than those on the other three surfaces during the early stance phase, which is consistent with that ankle plantar flexion muscle excitations increased following stepping on compliant surfaces ( Marigold and Patla, 2005 ).…”